1. Field of the Invention
The present invention relates to vibratory seismic energy sources used for seismic prospecting. More particularly, it relates to a system for synchronizing operation of the hydraulic lift cylinders and thus coordinating movement of the guide rods of a seismic vibrator.
2. Description of the Prior Art
In seismic prospecting, it has become common to use, as a source of elastic waves, an electrohydraulically operated vibratory source, more simply referred to as a vibrator.
Typically, a vibrator comprises a double ended piston rigidly affixed to a coaxial piston rod. The piston is located in reciprocating relationship in a cylinder formed within a heavy reaction mass. Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder, thereby imparting a reciprocating motion to the piston relative to the reaction mass. The piston rod extending from the reaction mass is rigidly coupled to a ground coupling plate (hereinafter baseplate) which is maintained in intimate contact with the earth material. The inertia of the reaction mass tends to resist displacement of the reaction mass relative to the earth. The motion of the piston is coupled through the piston rod and baseplate to impart vibratory seismic energy in the earth.
Typically, the vibrator is vehicle mounted. The baseplate is retractably positioned beneath the vehicle and is raised and lowered by a vibrator lift system. To prevent decoupling of the baseplate from the ground during operation a portion of the vehicle's weight is applied to the baseplate through a supporting means. The supporting means supports the weight of the vehicle, and may consist of a support structure positioned above the baseplate and one or more spring members affixed between the support structure and the baseplate. The vibrator lift system raises and lowers the vehicle so as to apply at least a portion of the vehicle's weight to the baseplate. The vibrator lift system usually comprises a pair of hydraulic lift cylinders, a pair of guide rods and a synchronizing system. The hydraulic lift cylinders control the vertical position of the baseplate relative to the vehicle and apply the weight of the vehicle to the baseplate. The guide rods laterally maintain the baseplate relative to the center of the vehicle. The synchronizing system insures the coordinated movement of the guide rods relative to the vehicle.
Various systems have been used to insure that the guide rods move up and down in unison. Where the guide rods have extended above the vibrator, they have been mechanically interconnected by a rigid stress member so as to coordinate their movement. Another mechanical synchronizing system (See U.S. Pat. No. 3,929,206, Bedenbender et al, issued Dec. 30, 1975) consists of a chain and sprocket arrangement interconnecting the guide rods, wherein the chain from one guide rod runs over a sprocket at one end of a synchronization shaft and the chain from the other guide rod runs over a sprocket at the opposite end of this shaft. An unequal force on one of the guide rods produces through its chain and sprocket arrangement a moment on the synchronization shaft which is transferred through the chain and sprocket arrangement of the other rod to equalize the load bearing force between guide rods.
A type of hydraulic system which obviates the necessity of mechanically interconnecting the guide rods has also been used to equalize travel between the guide rods. (See U.S. Pat. No. 3,306,391, Bays, issued Feb. 28, 1967). This system is comprised primarily of two double-rod-end hydraulic synchronizing cylinders disposed on opposite sides of the vibrator. On either side of the vibrator, a cross-piece extends from the guide rod and mechanically interconnects the synchronizing cylinder and the hydraulic lift cylinder. Nonanalogous chambers of the synchronizing cylinders are connected in fluid communication. The force exerted by the hydraulic lift cylinder is transferred through the cross-piece to the synchronizing cylinder. An unequal force exerted on one hydraulic lift cylinder will cause transfer of hydraulic fluid from one synchronizing cylinder to the other. Movement of either of the pistons of the synchronizing cylinders will cause a corresponding movement in the opposite synchronizing cylinder; thus, movement of either of the guide rods will of necessity result in a corresponding movement of the other guide rod.